Electronic Support (ES) and Radar Warning Receiver (RWR) systems need to precisely find Angles of Arrival (AoA) of RF emitters, including frequency agile emitters, as rapidly as possible in order to precisely determine direction. As known, direction finding is important in supporting jamming and other tactical applications. For pairs of antennas spaced many wavelengths apart, phase interferometry is capable of highly precise AoA measurements with less than 1° error, but multiple ambiguous angles of arrival are computed, caused by the periodic nature of the phase difference related to the geometric angle. Antennas spaced less than one-half wavelength apart have less precise AoA measurements. Many ES and RWR systems use a single antenna and receiver to cover a broad spectrum, e.g., 2-18 GHz, so antenna spacing at one-half of the largest wavelength may represent many multiples of the smallest wavelength. Further, many platforms are already outfitted with antennas spaced many wavelengths apart to provide coverage at all azimuths about the platform.
Time Difference of Arrival (TDOA) direction finding may be used to compute AoA over a span of 180° without ambiguity. The time delay between two antennas is monotonic with respect to geometric angle changing from 0° to 90° (and conversely from −90° to 0°), but TDOA provides relatively low AoA precision. Amplitude comparison direction finding generally provides a similarly coarse AoA precision. Improved AoA precision may be achievable using Frequency Difference of Arrival (FDOA) techniques, however FDOA requires stable emitter signal frequencies for long periods of time, e.g., 10ths of seconds, seconds, or many seconds, depending on platform velocity and desired AoA precision, thus limiting the number of threats that can be simultaneously geolocated and limiting applicability to threats with a constant frequency.
In comparison, an interferometer can provide precision AoA in a single pulse time, e.g., microseconds to milliseconds. Interferometers in ES (RWR) systems, however, require a number of receivers to provide unambiguous Angle of Arrival (AoA). Receivers can be costly in dollars or power/thermal requirements, especially when they must be mounted on the exterior of an airborne platform. As receiver cost is proportional to bandwidth/data rate and the trend is to increase receiver bandwidth to deal with increasingly wideband and agile emitters, the costs have increased significantly.
A prior attempt to address the use of interferometers for AoA determination implemented a method to cover the entire spectrum of interest in one mode using N receivers but with a lower average data rate and then to switch the same receivers to perform interferometry while being vigilant over the whole spectrum.
What is needed is a further improvement to interferometry for determining a location of an RF emitter.